Field of the Invention
The present invention relates to passivating the surface of crystalline iron disulfide (FeS2) by encapsulating it in crystalline zinc sulfide (ZnS). It also relates to FeS2 encapsulated by ZnS in which the sulfur atoms at the FeS2 surfaces are passivated. Additionally, this invention relates to a photovoltaic (PV) device incorporating FeS2 encapsulated by ZnS.
Description of the Prior Art
Iron disulfide has great promise as an Earth-abundant material for PV applications. In a recent survey of 23 known semiconductor systems with potential as PV absorbers, FeS2 ranked highest in potential annual electricity production based on known reserves and had the lowest extraction cost. Wadia et al., “Materials availability expands the opportunity for large-scale photovoltaics deployment,” Environ. Sci. Technol., 43, pp. 2072-2077 (2009). Its bandgap is 0.95 eV, high enough to result in a potential solar to electricity conversion efficiency similar to that of Si, but unlike Si, it has an exceptionally high absorption coefficient of α=6×105 cm−1 resulting in a required thickness of <40 nm for >90% absorption (Ennaoui et al., “Iron disulfide for solar energy conversion,” Sol. Energ. Mat. Sol. Cells, 29, pp. 289-370 (1993)) compared to typical thicknesses >100 μm for Si.
Despite these apparent advantages, FeS2 PV devices have not yet lived up to their potential. Efficiency has been limited to approximately 3% due to open circuit voltage (Voc) of <200 mV, only ˜20% of the bandgap. Ennaoui et al., “Iron disulfide for solar energy conversion,” Sol. Energ. Mat. Sol. Cells, 29, pp. 289-370 (1993) and Wilcoxon et al., “Strong quantum confinement effects in semiconductors: FeS2 nanoclusters,” Sol. State Comm., 98, pp. 581-585 (1996). These limits have been shown to be the direct result of surface termination of FeS2 crystals. Bulk FeS2 crystallizes in the cubic pyrite structure, and its sulfur atoms are paired in an S—S bond (S22-). Crystal surfaces, however, are typically terminated by S monomers (S1-) that may convert to S2- through a redox reaction. Bi et al., “Air stable, photosensitive, phase pure iron pyrite nanocrystal thin films for photovoltaic application,” Nano Lett., 11, pp. 4953-4957 (2011) and Zhang et al., “Effect of surface stoichiometry on the band gap of the pyrite FeS2(100) surface,” Phys. Rev. B, 85, art. 085314 (2012). The resulting surface states exhibit a high density of defects within the FeS2 bandgap and shows properties similar to the iron monosulfide phase, with a bandgap of approximately 0.3 eV. In PV devices, these surface states at films' surfaces and grain boundaries lead to high dark current and low Voc.